1. Field of the Invention
The present invention is directed to a computer tomography apparatus, and in particular to a computer tomography apparatus having a row of primary radiation detectors with a row of scattered radiation detectors disposed adjacent thereto in the axial direction of the apparatus.
2. Description of the Prior Art
Computer tomography devices are known which produce transverse tomograms of an examination subject. Such devices have an x-ray source which generates a fan-shaped x-ray beam which penetrates an examination subject. The cross-sectional extent of the x-ray beam in the slice plane is equal to the desired slice thickness. Primary radiation attenuated by the examination subject, as well as scatter radiation, are generated by the passage of the x-radiation through the examination subject. A rotatory mechanism rotates the x-ray source and the detector rows around an axis of rotation so that the examination subject is irradiated by x-rays from different directions. The outputs of the detector rows are supplied to a computer, which constructs an image of the desired slice of the examination subject from the detector signals. The scatter radiation detected by the second row of detector elements is used to correct the signals of the row of primary radiation detector elements.
So-called "third generation" computer tomography devices have a detector row which is rigidly connected to the x-ray source, and which rotates around the examination subject. The detector elements of the detector row can be shielded against scatter radiation generated by the examination subject. Such shielding is accomplished by collimator plates directed at the focus of the x-ray source, and consisting of highly radiation-absorbent material. These collimator plates, given imperfect alignment of the x-ray source and the collimator plates or given fluctuations of the focal position due to thermal influences, cause a shadow to be cast on the detector elements, thereby resulting in a geometrical filling degree below the nominal grid dimension of the detector row during actual operation.
So-called "fourth generation" computer tomography devices have a stationary detector ring (or partial ring) which surrounds the measuring field in which the examination subject is disposed. In these types of computer tomography devices, collimation of the detector elements is not possible, or would be so technologically complex as to be impracticable. Even if collimation could be accomplished in fourth generation computer tomography devices, the same diminution in the filling factor would result.
It is known that a measurement of the scatter radiation outside of the actual tomograph slice correlates, with a good approximation, to the actual scatter radiation measured within the slice.
A computer tomography apparatus is described in German Pat. No. OS 26 42 846, corresponding to U.S. Pat. No. 4,149,080, wherein the scatter radiation outside of the actual tomograph slice is measured by a second detector row, and the output signals of the second detector row are used in the construction of the displayed image of the examination slice.